1. Field of the Invention
The present invention relates generally to an optical fiber doped with an alkali metal oxide and methods and apparatus for making same.
2. Technical Background
Attenuation is a principal limiting attribute of optical fibers. Optical fiber loss, for example, plays an important role in setting the limiting distance between optical fiber amplifiers. This is particularly important in long distance and ultra-long distance networks such as, for example, undersea applications, where such amplifiers represent a significant system cost, as well as a major factor in system reliability. Consequently there is tremendous commercial interest in reducing attenuation to the lowest possible level. Summary of the Invention
One broad aspect of the present invention relates to an optical fiber having a core comprising an alkali metal oxide selected from the group consisting of K2O, Na2O, LiO2, Rb2O, Cs2O and mixtures thereof, in a peak concentration greater than about 0.001 wt. % and less than about 1 wt. %; a cladding comprising the alkali metal oxide in a peak concentration less than the peak concentration in the core but greater than about 0.0005 wt. %; and wherein the concentration of alkali metal oxide varies with a radius of the optical fiber. The alkali metal oxide dopant concentration preferably decreases with increasing radius from the centerline of the optical fiber. Using the alkali metal oxide doping techniques disclosed herein, optical fibers can be made which exhibit an attenuation less than about 0.30 dB/km at 1310 nm and less than about 0.18 dB/km at 1550 nm; preferably less than about 0.17 dB/km at 1550 nm, more preferably less than about 0.16 dB/km at 1550 nm.
Preferably, both the core and the cladding of the optical fiber contain an alkali metal oxide dopant. The cladding glass of the optical fiber may comprise fluorine (F). The optical fiber has at least one core segment; in some preferred embodiments, the optical fiber comprises multiple core segments. The alkali metal oxide concentration at a radius equal to the mode field radius of the optical fiber is preferably at least about 0.001 wt. %.
The present invention proposes an optical fiber having a core comprising an alkali metal oxide selected from the group consisting of K2O, Na2O, LiO2, Rb2O, Cs2O and mixtures thereof, wherein the core contains less than 20 ppb of OH.
According to another aspect of embodiments of the invention, an optical fiber is proposed having a core comprising an alkali metal oxide selected from the group consisting of Rb2O, Cs2O and mixtures thereof, in a peak concentration greater than about 0.001 wt. % and less than about 1 wt. %, a cladding comprising the alkali metal oxide in a peak concentration less than the peak concentration in the core, but greater than about 0.0005 wt. %, and wherein the concentration of alkali metal oxide varies with a radius of the optical fiber.
According to still another aspect of embodiments of the invention, an optical fiber is proposed comprising a core containing Rb2O in a peak concentration greater than about 0.001 wt. % and less than about 1 wt. %, a cladding comprising Rb2O in a peak concentration less than the peak concentration in the core, but greater than about 0.0005 wt. % and wherein the concentration of alkali metal oxide varies with a radius of the optical fiber.
According to another broad aspect of the present invention, an optical fiber may be provided comprising a core comprising GeO2 and an alkali metal oxide selected from the group consisting of K2O, Na2O, LiO2, Rb2O, Cs2O and mixtures thereof, and wherein a refractive index of the optical fiber is selected to provide a total dispersion greater than about 1 ps/nm/km at about 1550 nm, and a dispersion slope less than about 0.10 ps/nm2/km at 1550 nm. Preferably, the optical fiber has a total dispersion greater than about 6 ps/nm/km at 1550 nm. Preferably, the optical fiber has an attenuation less than about 0.18 dB/km at 1550 nm; more preferably less than about 0.17 dB/km at 1550 nm. Preferably, the optical fiber is drawn at a draw speed of at least 10 m/s.
According to another aspect of the invention, an optical fiber is disclosed herein comprising: a silica-based core comprising a first dopant selected from the group consisting of germania and fluorine and mixtures thereof, and an alkali metal oxide selected from the group consisting of K2O, Na2O, LiO2, Rb2O, Cs2O and mixtures thereof, in a peak concentration between 20 and 1000 ppm; and a silica-based cladding surrounding and directly adjacent the core; wherein the attenuation at 1550 nm is less than 0.185 dB/km, preferably less than 0.18 dB/km, more preferably less than 0.17 dB/km. In some preferred embodiments, the attenuation at 1550 nm is less than or equal to 0.167 dB/km. In preferred embodiments, the concentration of alkali metal oxide in the core decreases with a radius of the optical fiber. Preferably, the peak concentration of alkali metal oxide in the core is greater than about 0.002 wt.% and less than about 0.07 wt. %. In preferred embodiments, the alkali metal oxide concentration at a radius equal to a mode field radius of the optical fiber is at least about 0.0001 wt. %. In some embodiments, the core comprises GeO2, and in other embodiments, the core comprises no GeO2. In some embodiments, the core comprises a single segment. In other embodiments, the core comprises a plurality of segments. In some preferred embodiments, the cladding comprises F, particularly in some embodiments where the core has no germania. In preferred embodiments, the peak amount of alkali metal oxide in the core is greater than about 0.002 wt % and less than about 0.05 wt. %. In various embodiments, the optical fiber comprises an exterior hermetic coating; in particular embodiments, the first dopant is germania, i.e. the fiber is germania-doped, and the optical fiber further comprises an exterior hermetic coating. In some preferred embodiments, the optical fiber is a single mode fiber, for example single-moded at 1550 nm; in other preferred embodiments, the optical fiber is a multimode fiber, which preferably has a graded refractive index profile. Some preferred embodiments are non-zero dispersion shifted optical fibers having a dispersion at 1550 nm between 1 and 6 ps/nm-km, and other embodiments have a dispersion at 1550 nm between 6 and 15 ps/nm-km.
According to yet another aspect of the invention, an optical fiber is disclosed herein comprising: a core comprising GeO2 and an alkali metal oxide selected from the group consisting of K2O, Na2O, LiO2, Rb2O, Cs2O and mixtures thereof; and a cladding surrounding the core, wherein a refractive index profile of the optical fiber is selected to provide a total dispersion greater than about 1 ps/nm/km at 1550 nm, and a dispersion slope less than about 0.10 ps/nm2/km at the zero dispersion wavelength. In preferred embodiments, the total dispersion is greater than about 6 ps/nm2/km at 1550 nm. Preferably, the attenuation at 1550 nm less than about 0.18 dB/km, more preferably less than about 0.17 dB/km.
In another broad aspect of the invention, an optical fiber is disclosed herein comprising: a core comprising an alkali metal oxide selected from the group consisting of Rb2O and Cs2O and mixtures thereof, in a peak concentration greater than about 0.001 wt.% and less than about 1 wt. %; and a cladding surrounding and directly adjacent the core.
In still another broad aspect of the invention, an optical fiber is disclosed herein comprising: a core comprising Rb2O in a peak concentration greater than about 0.001 wt. % and less than about 1 wt. %; and a cladding surrounding and directly adjacent the core.
In another broad aspect of the invention, an optical fiber is disclosed herein comprising: a silica-based core comprising a first dopant selected from the group consisting of germania and fluorine and mixtures thereof, and an alkali metal oxide selected from the group consisting of K2O, Na2O, LiO2, Rb2O, Cs2O and mixtures thereof, in a peak concentration between 20 and 1000 ppm; and a silica-based cladding surrounding and directly adjacent the core; wherein the core has a refractive index profile with a peak relative refractive index, ΔMAX, greater than 0.2%, relative to the cladding. Preferably, the optical fiber has an attenuation at 1550 nm of less than 0.185 dB/km, more preferably less than 0.18 dB/km, even more preferably less than or equal to 0.17 dB/km. In some preferred embodiments, the attenuation at 1550 nm is less than or equal to 0.167 dB/km. In some preferred embodiments, the fiber is a multimode fiber and the core comprises at least 70 wt % SiO2. In other preferred embodiments, the core comprises at least 80 wt % SiO2. In still other preferred embodiments. the core comprises at least 90 wt % SiO2. Preferably, the optical fiber is a single-mode fiber and the core comprises at least 90 wt % SiO2. Preferably, the core further comprises chlorine in a peak concentration of less than 3000 ppm. Preferably, the peak concentration of the alkali metal oxide is less than 700 ppm. Preferably, the average concentration of the alkali metal oxide is less than 350 ppm. In some preferred embodiments, the peak concentration of the alkali metal oxide is less than 500 ppm, that is the peak concentration of the alkali metal oxide is between 20 and 500 ppm. In preferred embodiments, the alkali metal oxide is K2O. In a first set of preferred embodiments, the first dopant is germania and the peak concentration of the alkali metal oxide is between 30 and 300 ppm, preferably between 30 and 150 ppm. The core preferably further comprises chlorine in a peak concentration less than 3000 ppm. Preferably, the core has a maximum concentration of fluorine of less than 0.2 wt %. In some preferred embodiments, the cladding comprises an alkali metal oxide selected from the group consisting of K2O, Na2O, LiO2, Rb2O, Cs2O and mixtures thereof, in a peak concentration of less than 100 ppm. In a second set of preferred embodiments, the first dopant is fluorine and the peak concentration of the alkali metal oxide is between 200 and 500 ppm, and some preferred embodiments is between 100 and 300 ppm. Preferably, the core has a concentration of fluorine of greater than 0.02 wt %, even more preferably the core has a concentration of fluorine of greater than 0.02 wt % at the centerline. Preferably, the core has a concentration of fluorine of greater than 0.15 wt %. Preferably, the core has a maximum concentration of fluorine of between 0.5 and 1.5 wt %. In particularly preferred embodiments of the second set, the core contains essentially no germania, preferably no germania. Preferably, the cladding has a minimum concentration of fluorine of at least 1.0 wt %. In preferred embodiments, the alkali metal oxide is K2O. In some embodiments, the core further comprises chlorine in a peak concentration less than 500 ppm. Preferably, the cladding comprises an alkali metal oxide selected from the group consisting of K2O, Na2O, LiO2, Rb2O, Cs2O and mixtures thereof, in a peak concentration of less than 100 ppm.
An optical fiber preform is disclosed herein having a center portion consisting essentially of solid glass, the center portion being surrounded by an outer portion comprised of glass soot, wherein the center portion contains an alkali metal oxide selected from the group consisting of K2O, Na2O, LiO2, Rb2O, Cs2O and mixtures thereof. Preferably, the alkali metal oxide is selected from the group consisting of K2O and Rb2O. Preferably, the center portion also contains GeO2. The outer portion preferably comprises GeO2. The center portion preferably contains less than 20 ppb OH.
In still another broad aspect of the present invention, a method of making an optical fiber is disclosed comprising forming a first glass rod comprising an alkali metal oxide selected from the group consisting of K2O, Na2O, LiO2, Rb2O, Cs2O and mixtures thereof, and inserting the first glass rod into a centerline hole of an optical fiber preform to form a composite preform assembly. In one preferred embodiment, the glass rod comprises GeO2. Preferably, the optical fiber preform comprises GeO2. At various points in its manufacture, the optical fiber preform preferably comprises a glass soot.
Yet another broad aspect of the invention involves a method of making an optical fiber comprising providing an optical fiber preform comprising an alkali metal oxide selected from the group consisting of K2O, Na2O, LiO2, Rb2O, Cs2O and mixtures thereof, and drawing the optical fiber preform into an optical fiber, wherein the draw speed and the draw tension are selected to control a concentration of alkali metal oxide in the optical fiber, and wherein the concentration varies with radius.
Another broad aspect of the invention provides for a method of making an optical fiber comprising the steps of providing an optical fiber preform comprising an alkali metal oxide selected from the group consisting of K2O, Na2O, LiO2, Rb2O, Cs2O and mixtures thereof, and heat treating the optical fiber preform for a time and at a temperature effective to obtain a pre-determined concentration of the alkali metal oxide in the optical fiber preform as a function of radius. Preferably, the method includes heat treating the optical fiber preform for at least about 6 hours. The optical fiber preform is heat treated preferably at a temperature of at least 1000° C. Preferably, a cladding glass of the optical fiber preform comprises F.
In accordance with another broad aspect, the invention provides for a method of making an optical fiber comprising the steps providing a glass article having an outer dimension (dl) and doped with an alkali metal oxide selected from the group consisting of K2O, Na2O, LiO2, Rb2O, Cs2O and mixtures thereof; and adding additional glass to the glass article to form a final consolidated draw preform having a final outer dimension (d2), wherein the outer dimension (d1) is less than or equal to 0.06 times the final outer dimension (d2) thereby concentrating the alkali metal oxide near the center of the final consolidated draw preform.
In accordance with another broad aspect, the invention provides for a method of making an optical fiber comprising the steps of depositing silica-containing soot onto a rotating mandrel to form a silica-containing soot tube, first drying the silica-containing soot tube with a chlorine-containing gas, then further drying the silica-containing soot tube with a fluorine-containing gas, consolidating the silica soot tube to form a glass tube, doping the glass tube or an intermediate article formed from the glass tube with an alkali metal oxide selected from the group consisting of K2O, Na2O, LiO2, Rb2O, Cs2O and mixtures thereof; collapsing the glass tube or intermediate article to form an alkali-doped rod, and adding additional silica-containing glass onto the alkali-doped rod.
In accordance with a further broad aspect, the invention provides for a method of making an optical fiber comprising the steps of depositing silica-containing soot onto a rotating mandrel to form a silica-containing soot tube, drying the silica-containing soot tube with a chlorine-containing gas, further drying the silica-containing soot tube with a fluorine-containing gas, consolidating the silica soot tube to form a glass tube, doping the glass tube or an intermediate glass article formed from the glass tube with an alkali metal oxide selected from the group consisting of K2O, Na2O, LiO2, Rb2O, Cs2O and mixtures thereof; collapsing the glass tube or intermediate article to form an alkali-doped rod, inserting the alkali-doped rod into a silica-containing soot tube, forming a core rod from the alkali-doped rod and silica-containing soot tube, adding fluorine-doped silica to the core rod, and consolidating the fluorine-doped silica to form a final draw perform.
Further, and in accordance with another broad aspect, the invention provides for a method of making an optical fiber comprising the steps of depositing germanium-doped silica soot onto a rotating mandrel to form a germanium-doped silica soot tube, drying the germanium-doped silica soot tube with a chlorine-containing gas, further drying the silica-containing soot tube with a fluorine-containing gas, consolidating the germanium-doped silica soot tube to form a glass tube, doping the glass tube or a intermediate article formed from the glass tube with an alkali metal oxide selected from the group consisting of K2O, Na2O, LiO2, Rb2O, Cs2O and mixtures thereof; forming an alkali-doped rod from the glass tube or the intermediate article, and inserting the alkali-doped rod into a silica-containing soot tube, the silica-containing soot tube including a inner annular portion of germanium-doped silica soot and an outer annular portion of substantially undoped silica soot.
In accordance with another broad aspect, the invention provides for a method of making an optical fiber comprising the steps of depositing silica-containing soot onto a rotating mandrel to form a silica-containing soot tube, drying the silica-containing soot tube with a chlorine-containing gas, further drying the silica-containing soot tube with a fluorine-containing gas, consolidating the silica-containing soot tube to form a glass tube, doping the glass tube or an intermediate article formed from the glass tube with an alkali metal oxide selected from the group consisting of K2O, Na2O, LiO2, Rb2O, Cs2O and mixtures thereof to form an alkali-doped article wherein the alkali metal oxide is doped in an amount of between about 20-1000 ppm of the alkali metal oxide.
In accordance with another broad aspect, the invention provides a diffusion doping apparatus, comprising a frame, a glass tube mounted for rotation relative to the frame, a source of dopant coupled to the glass tube, and an induction heater mounted proximate to the glass tube.
Additional features and advantages of the invention will be set forth in the detailed description which follows, and in part will be readily apparent to those skilled in the art from that description or recognized by practicing the invention as described herein, including the detailed description which follows, the claims, as well as the appended drawings.
It is to be understood that both the foregoing general description and the following detailed description present embodiments of the invention, and are intended to provide an overview or framework for understanding the nature and character of the invention as it is claimed. The accompanying drawings are included to provide a further understanding of the invention, and are incorporated into and constitute a part of this specification. The drawings illustrate various embodiments of the invention, and together with the description serve to explain the principles and operations of the invention. Where appropriate, identical features have been identically numbered.